This invention relates to a process for the purification of a polyether polyol that was produced with an alkaline metal hydroxide catalyst. It also relates to the reuse of the recovered alkaline catalyst in the subsequent manufacture of polyether polyols. The polyether polyols produced by this process are also the subject of this invention.
Polyether polyols are manufactured commercially using the base catalyzed reaction of initiators having active hydrogen atoms with epoxides such as, for example, ethylene oxide and propylene oxide. Alkalinity is introduced into the polyether polyols, for example, by using metal hydroxides as catalysts. Virtually any strong base can be used as the catalyst for the manufacture of polyether polyols. Some examples of strong bases suitable for use as catalysts include metal alkoxides of low boiling alcohols (e.g., potassium methoxide, potassium ethoxide, etc.), or even the pure alkali metals (potassium or sodium metal). The use of these with a hydroxyl group containing initiator, ROH, is as shown in Equations I and II below. EQU CH.sub.3 O.sup.- K.sup.+ +ROH.fwdarw.RO.sup.- K.sup.= =CH.sub.3 OH.uparw.(I ) EQU K.sub.(metal) +ROH.fwdarw.RO.sup.- K.sup.+ +1/2H.sub.2 .uparw.(II)
In equation (I), the methanol formed can be distilled off driving the reaction to the right.
Potassium hydroxide (KOH) and sodium hydroxide (NaOH) are some examples of typical alkaline catalysts used. In general, the metal hydroxide catalyst is added to the initiator (usually a hydroxyl group containing compound), and an equilibrium between the metal hydroxide and the initiator occurs. This equilibrium is as shown is Equation III below. EQU KOH+ROH=H.sub.2 O+RO.sup.- K.sup.+ (III)
Both the hydroxide and the alkoxide can react with epoxides. This is often acceptable for short chain (low equivalent weight) polyols, but the reaction of water is undesirable in the preparation of long chain (i.e., high equivalent weight) polyols. It is therefore, necessary to force the above equilibrium to the right by removing the water (i.e., dewatering). This converts all of the hydroxide to alkoxide. The total amount of alkalinity remains constant and is equal to the amount of KOH originally added.
The alkalinity concentration is measured and reported as percent KOH. This percent KOH actually reflects the alkalinity which is present, even though this alkalinity is not necessarily KOH. Rather, it may actually be NaOH or another alkaline species such as an alkoxide.
Once the polymerization of the epoxide(s) is completed, the alkaline catalyst must be neutralized and/or removed from the crude mixture to yield the final polyether polyol. Several processes for the removal of the residual catalysts from the crude polyether polyols to yield the final product are known.
Known processes for removing the residual alkaline catalysts from polyether polyols are costly, time consuming and wasteful. One process for removing residual sodium or potassium hydroxide from polyether polyols comprises neutralizing the basic material (hydroxide or alkoxide) with aqueous sulfuric acid, distilling to remove water, and then filtering to remove the solid sodium or potassium sulfate salt which is formed by this process. This particular process results in additional costs related to the KOH and the sulfuric acid raw materials used and disposal of the filtercake which is formed. It also results in a loss of yield with respect to the polyether polyol as some polyether polyol inevitably remains in the filtercake unless additional steps are taken to recover it.
Another process for purifying polyether polyols is described in U.S. Pat. No. 5,449,841. This process specifically relates to reducing the level of metal ions and/or metal compounds of polyoxyalkylene monool and/or polyols having number average molecular weights of above 500 up to 25,000 by bringing these monools and/or polyols into contact with an extracting compound which is a polyol or polyol mixture having a number average molecular weight of at most 500, preferably at most 250, and mixing the extracting compound and the polyoxyalkylene monool or polyol. These extracting compounds must be immiscible with the polyoxyalkylene monool or polyol. The mixture of extracting compound and the polyoxyalkylene monool or polyol are allowed to separate, and the liquid extracting compound containing the alkaline catalyst is removed.
Glycerine is broadly disclosed as a polyol to be used as a suitable extracting compound in U.S. Pat. No. 5,449,841. The most preferred extracting compounds include, however, ethylene glycol, diethylene glycol and mixtures thereof. Diethylene glycol is used as the extracting compound in the only example. Diethylene glycol is not suitable for the presently claimed invention.
According to U.S. Pat. No. 5,449,841, the quantity of extracting compound preferred by the process therein is at least 25 parts extracting compound per 100 parts monool or polyol, and more preferably from 30 parts to 500 parts. This amount is, however, substantially higher than that required by the presently claimed invention. Furthermore, this reference clearly discloses that these liquid-liquid extractions form two separate layers upon standing at room temperature which can be separated. After separating these two layers, these polyols are then subjected to vacuum distillation at high temperature for long periods of time to remove any residual extracting compound. There is no information that suggests this process enables the reuse and/or recovery of the extracting compound or alkaline catalyst.
Other processes are also known in the art for removing alkaline catalysts from polyether polyols. Some examples include the use of ion exchange resins as described in, for example, U.S. Pat. No. 4,985,551; extraction with water as described in, for example, Canadian Patent Application 2,165,140; and the use of lactic acid as described in, for example, U.S. Pat. No. 4,430,490.
Accordingly, a need exists for a process by which most of the alkaline catalyst can be easily removed and, preferably be reused to prepare subsequent batches of polyether polyols. Such a process would assist in reducing costs associated with the manufacture of polyether polyols.
An object of the present invention was to provide a simple and efficient means of removing the alkaline catalyst from a polyether polyol to provide a substantially neutral product. A second object of the present invention was to provide a means to reuse the catalyst in subsequent batches of polyether polyols without undergoing elaborate concentration or purification steps.
In accordance with the present invention, the reaction of the present process is as shown in Equation (IV) below. EQU RO.sup.- K.sup.+ +glycerine.fwdarw.ROH+glycerine.sup.- K.sup.+ (IV)
In U.S. Pat. No. 5,449,841, the equilibrium is based on the relative acidities of the two alcohols. In the present invention, however, since the potassium glycerinate is a solid, it precipitates and is removed from the equilibrium, thereby forcing the reaction to completion. This is advantageous from a recycling point of view thus making the distillation step required by previous processes when using alkaline metal hydroxides or alkoxides no longer necessary.